Components used to convey fuel in motor vehicle fuel systems and other applications that require contact with fuels have traditionally been made from metals. However, it often can be desirable to make such components from polymeric materials because of their light weight and ability to be formed into intricate parts. The use of polymeric materials can provide significant flexibility in part design change, as mold designs can be altered. Polymeric materials can be formed into seamless articles that are less likely to leak than articles having seams. Polymeric materials suitable for use in fuel systems should be sufficiently electrically conductive to dissipate any static electricity that might build up and thermally stable during processing and use (as they will in many cases be used in engine compartments and other areas having elevated temperatures or be exposed to fuels having elevated temperatures). It is also desirable that a polymeric material used in a fuel system be chemically resistant to the fuels with which they come into contact.
Polyacetal (also called polyoxymethylene) resins exhibit excellent mechanical and physical properties, such as tensile strength, stiffness, as well as fatigue resistance, sliding resistance, chemical resistance, and the like. The resins are used extensively in various applications as an engineering plastic material due to their excellent physical properties (such as mechanical and electrical properties) and chemical resistance. However, conventional polyacetal compositions can degrade when exposed to certain fuels, including diesel fuels; peroxide-containing fuels; and fuels containing toluene and isoheptane, particularly at elevated temperatures. Thus conventional polyacetals must be stabilized to avoid such degradation. Many additives that can be used to stabilize polyacetal compositions to degradation caused by contact with fuels can also lead to decreased stability, and thus degradation, of the compositions during processing. Additionally, many additives that can be used to provide polyacetal compositions that are capable of dissipating static electricity (that is, render POM compositions static dissipative) can also lead to decreased stability of the compositions during processing. In addition, the use of these types of additives can deleteriously affect the physical properties of the compositions. Furthermore, stabilizer additives can migrate to the surface of molded parts, leading to blooming, which can have a deleterious affect on the surface appearance of such parts.
It would thus be desirable to obtain a static dissipative polyacetal composition having a combination of good stability upon exposure to fuels, good stability during processing, good surface appearance in molded articles, and good physical properties.
EP 0 855 424 B1 describes the use of a sterically hindered amine compound to improve the diesel fuel resistance of molded parts many from polyoxymethylenes.